The invention relates to pneumatic devices and especially to compressed air driven tools. More particularly, the invention provides a compressed air driven bucking bar for use in forming joints with rivets. The invention provides a reduced recoil bucking bar that operates more efficiently and is less fatiguing to the user in comparison with previous devices.
Rivets are commonly used to form strong and secure joints between parts in buildings, aircraft, and numerous other structures and machines. To form a riveted joint, the two parts to be joined are brought together and a rivet is placed through the two parts through a common predrilled rivet hole. The rivet has a rounded head that bears against one of the parts, and an elongate shank that projects out of the rivet hole on the other side of the joint.
A pneumatic riveting hammer is pressed against the rivet on one side of the joint, usually against the head of the rivet. At the same time, a bucking bar is held against the shank of the rivet on the other side of the joint. When the hammer is actuated, it delivers a series of sharp impacts to the head of the rivet. These impacts send a shock wave down the length of the shank to the bucking bar at the other end.
When the shock wave reaches the end of the rivet, a portion of the energy is retained in the rivet, with the remaining portion transferred to the bucking bar. The energy retained in the rivet is absorbed in part by deforming the rivet. As the rivet deforms, a rounded head is formed on the shank to retain the rivet in the hole and secure the two parts together.
Of the energy transferred to the bucking bar, a portion is transferred back into the rivet, and another portion is absorbed by the bucking bar itself. The remaining energy flows into the hands of the worker holding the bucking bar, principally in the form of shock and vibration.
Formerly, solid metal bars were used as bucking bars. Unfortunately, relatively little energy was absorbed or returned to the rivet by the solid bar. Thus, a relatively large portion of the energy entering the bucking bar had to be absorbed by the worker. Not only did the worker holding the bar fatigue quickly, but the large shock and vibration amplitudes made it difficult to hold the bucking bar in place against the rivet.
For more efficient rivet forming, it is desirable to increase the proportion of energy returned from the bucking bar back into the rivet. This increases the rate at which the shank is deformed to form the rounded head on the rivet so that riveting is accomplished more quickly. To decrease the fatigue experienced by the worker using the bucking bar, the amount of energy transferred to the worker should be reduced. This can be accomplished by increasing the amount of energy returned to the rivet from the buckling bar, and by increasing the amount of energy absorbed by the bucking bar.
Efforts have been made to devise improved bucking bars which would reduce shock and vibration experienced by the worker while allowing more rapid and efficient riveting. For example, U.S. Pat. No. 2,512,532 to Sargent et al and U.S. Pat. No. 2,519,308 to Brown each describe bucking bars in which the shock wave from riveting acts on a mass that in turn compresses one or more springs within a cylindrical handle held by the worker. Compressing the spring stores energy; a portion of this energy is returned to the rivet when the spring rebounds the mass back against the rivet. Additionally, some energy is absorbed by the bucking bar in the form of heat produced in compressing the spring, and in moving the mass against frictional resistance in the cylinder.
A somewhat different approach is taken in the bucking bar described in U.S. Pat. No. 4,380,923 to Emmerich. A connection is provided between the bucking bar and an external supply of pressurized air. The air is used to pressurize an internal chamber that lies behind a piston connected to the impact head of the bucking bar. When a blow is delivered by the hammer to the rivet, energy passing from the rivet into the bucking bar forces the piston backward, thereby compressing the pressurized air in the pressure chamber. The air acts as a spring storing energy for return to the rivet as the piston recoils from the blow. Additionally, some energy is absorbed in compressing the air, and in sliding the piston against frictional resistance.
The device disclosed in the '923 patent includes a knob for adjusting the pressure of the air in the pressure chamber behind the piston. The '923 patent discloses that by turning the knob, the pressure in the chamber can be adjusted to minimize the vibration experienced by the worker. The '923 patent suggests that the knob be adjusted by the worker mainly on the basis of trial and error, with adjustments being made manually on the basis of a test run and from time-to-time as the riveting operation proceeds.
It would be desirable to devise a bucking bar that would automatically provide near optimal functioning under a wide variety of conditions, without manual adjustment or other intervention being required of the worker using the bar. It would be desirable if the bucking bar could in some way "sense" the impact transferred from the rivet and adjust itself so as to provide the proper resistance and recoil force. It would be further desirable to devise a bucking bar capable of absorbing an increased amount of energy, so that the energy delivered into the hands of the worker could be decreased accordingly.